The present disclosure relates to an optical scanning apparatus for irradiating a light beam and an image forming apparatus for forming an image using an electrostatic latent image formed by the irradiation of a light beam.
Image forming apparatuses such as printers and copiers often include a photosensitive drum that has a circumferential face for holding a toner image, and an optical scanning apparatus for irradiating a light beam on the circumferential face of the photosensitive drum. When the optical scanning apparatus irradiates a light beam on the circumferential face of the photosensitive drum, a potential difference arises on the circumferential face of the photosensitive drum, between areas being irradiated by the light beam from the optical scanning apparatus and areas not being irradiated. Accordingly, when the optical scanning apparatus emits a light beam in accordance with image data, an electrostatic latent image corresponding to the image data is formed on the circumferential face of the photosensitive drum. When toner is supplied onto the circumferential face of the photosensitive drum, on which the electrostatic latent image is formed, the toner becomes distributed according to the potential difference on the photosensitive drum. As a result, a toner image is formed on the circumferential face of the photosensitive drum. The toner image on the photosensitive drum is then electrostatically transferred to a sheet.
An optical scanning apparatus typically has a laser light source that emits a laser light beam, and a polygon mirror that rotates so that the laser light beam emitted from the laser light source moves in a main scanning direction. The position of the irradiation point of the laser light beam on the circumferential face of the photosensitive drum is defined in the main scanning direction according to the rotation angle of the polygon mirror.
In order to form an electrostatic latent image, the photosensitive drum may also rotate. The position of the irradiation point of the laser light beam on the circumferential face of the photosensitive drum is defined in a sub-scanning direction as a result of this.
The emission timing of the laser light beam may be adjusted according to the rotation positions of the polygon mirror and of the photosensitive drum. The optical scanning apparatus typically includes a light receiving element that receives the laser light beam reflected by the polygon mirror. If the polygon mirror is at a predetermined rotation position, the light receiving element receives the laser light beam. The laser light source emits the laser light beam at an appropriate time according to the light receiving timing of the light receiving element and the image data.
If the light receiving element appropriately receives the laser light beam, the start position of a writing line conforms to the sub-scanning direction in order to form an image.
If the light receiving element is located in a movement plane defined by the movement of the light beam according to the rotation of the polygon mirror, the light receiving element may be able to receive the laser light beam. However, if the light receiving element becomes separated from the movement plane, the light receiving element cannot receive the laser light beam. If the light receiving element does not receive the laser light beam, the image forming function of the image forming apparatus is impaired. As a result, the image forming apparatus may, for example, output a white sheet with nothing recorded thereon. Or, the image forming apparatus may output a sheet that has a black image formed on its entirety.